Method for producing esters of alpha, beta-unsaturated carboxylic acids

ABSTRACT

An ester of an α,β-unsaturated carboxylic acid is prepared by reacting the carboxylic acid with a C 6 -C 12 -alkanol, cyclopentanol or cyclohexanol in the presence of an acidic esterification catalyst and of an entraining agent for removing the water of reaction formed in the esterification, by a process in which the entraining agent used is the olefin on which the alkanol is based.

[0001] The present invention relates to a process for the preparation ofesters of an α,β-unsaturated carboxylic acid by reacting the carboxylicacid with an alkanol in the presence of an acidic esterificationcatalyst and in the presence of an entraining agent with which the waterof reaction formed during the esterification is removed from theesterification process.

[0002] The preparation of alkyl esters of α,β-unsaturated carboxylicacids is generally known. It is usually carried out by esterifying theacids with the corresponding alcohol at elevated temperatures in theliquid phase with or without a solvent and in the presence of a strongacid as a catalyst (cf. for example DE-A-23 39 519). In order to avoidpolymerization, polymerization inhibitors are generally added. Ofparticular industrial importance is the esterification of (meth)acrylicacid. The term (meth)acrylic acid denotes acrylic or methacrylic acid inthe known manner.

[0003] The disadvantage of the esterification processes of the prior artis that, under the esterification conditions, unconverted startingalcohol and unconverted carboxylic acid undergo addition at the doublebond of already formed alkyl carboxylate as secondary reactions (Michaeladdition). Multiple addition is also possible. Furthermore, mixed typesmay occur. These adducts are alkoxy esters and acyloxy esters and theyare referred to briefly as oxyesters. Typical of the oxyesters is thefact that their boiling point is above the boiling points of startingacid, starting alcohol, desired ester formed and any organic solventpresent. In the production of the desired ester, they are thereforeobtained as a residue and result in considerable reductions in yield.There have therefore been numerous attempts to recover at least a partof the compounds used or the desired ester from the oxyesters (cf. forexample DE-A-195 36 191 and the prior art mentioned therein).

[0004] In the cleavage process of DE 195 36 191, a considerable amountof olefins is formed, which cannot be further used in the esterificationprocess and therefore has to be separated off and removed.

[0005] A further disadvantage of the conventional esterificationprocesses is a consequence of the fact that the ester formation is basedon an equilibrium reaction. In order to obtain economical conversions,as a rule a starting material is used in excess and/or the water ofesterification formed and/or the desired ester are removed from theequilibrium. In order to shift the equilibrium in the direction of esterformation, an organic entraining agent which forms an azeotropic mixturewith water is frequently added. In particular the esterification withhigher alkanols is carried out in the presence of an entraining agentfor the water of reaction (Kirk-Othmer, “Encyclopedia of ChemicalTechnology”, Vol. 1, page 347, Ullmann's Encyclopedia of IndustrialChemistry, 5th Ed., 1985, Vol. A1, 168; U.S. Pat. Nos. 2,917,538,5,386,052). Preferably, inert hydrocarbons, e.g. cyclohexane, hexane,benzene and toluene, are used.

[0006] The addition of a further “foreign” solvent is howeverdisadvantageous since it has to be separated off separately and, ifnecessary, purified before being recycled.

[0007] DE 2 548 561 therefore proposes separating off the water ofreaction formed in the reaction of acrylic acid with ethylhexanol as anazeotropic mixture with ethylhexanol. According to the example,ethylhexanol is used in excess relative to acrylic acid (1:1.42). Thedisadvantage here is that the esterification has to be carried out underreduced pressure and a residue of about 8% by weight, based on thedesired ester, is obtained and has to be disposed of. The process thuscauses environmental pollution and is uneconomical.

[0008] Influencing the esterification equilibrium by using a largerexcess of a starting material (alkanol or carboxylic acid) leads to theformation of byproducts, e.g. ethers and olefins from the alkanol usedand acyloxyesters and alkoxyesters by Michael addition of the alkanol orof the carboxylic acid. This is described, for example for acrylic acid,in US-A-4,280,010 and in DE-A-2 339 519. These byproducts have to beseparated off and disposed of in an expensive manner, which isuneconomical and pollutes the environment.

[0009] Even without the use of a relatively large excess of alcohol,some of the esters and the alcohol are cleaved under the strongly acidicesterification conditions and olefins are formed, as described, forexample, in DE-A-195 36 191 and Houben-Weyl, Methoden der OrganischenChemie, Volume VIII/3, 1952, page 534.

[0010] Most processes of the prior art therefore have in common thedisadvantage that undesired byproducts have to be separated off and thatan additional solvent is required as an entraining agent for removingthe water of esterification.

[0011] It is an object of the present invention to provide anesterification process which is simple to carry out and in which theformation of undesired byproducts is reduced.

[0012] We have found, surprisingly, that this object is achieved if theolefin which is formed by the elimination of water from the alkanol usedfor the esterification is used as an entraining agent.

[0013] The present invention therefore relates to a process for thepreparation of an ester of an α,β-unsaturated carboxylic acid byreacting the carboxylic acid with an alcohol which is selected fromC₆-C₁₂-alkanols, cyclopentanol and cyclohexanol, in the presence of anacidic esterification catalyst and of an entraining agent for removingthe water of reaction formed in the esterification, wherein theentraining agent used is the olefin on which the alcohol is based, i.e.an olefin which corresponds to an olefin obtainable by eliminating waterfrom the alcohol employed is used.

[0014] With the exception of the entraining agent used, the novelesterification process is carried out in a conventional manner. Suitableprocesses are described, for example, in Ullmann's Encyclopedia ofIndustrial Chemistry, 5th Ed., 1985, Vol. A1, 168, 169; Kirk-Othmer,Encyclopedia of Chemical Technology, Vol. 1, pages 341-348; U.S. Pat.Nos. 2,917,538; 5,385,052. The processes are described using acrylicacid as an example, but the novel process is not restricted to acrylicacid. The esters of other α,β-unsaturated carboxylic acids can beprepared analogously.

[0015] The esterifications are typically carried out at from about 80 to160° C., preferably from 90 to 130° C., and in the presence of an acidicesterification catalyst, for example a mineral acid, a sulfonic acid orphosphoric acid. Sulfuric acid and sulfonic acids are particularlysuitable, especially p-toluenesulfonic, benzenesulfonic,dodecylbenzenesulfonic and methanesulfonic acid. The amount of catalystis from about 0.1 to 10, preferably from 0.5 to 5, % by weight, based onthe other starting materials. The esterification is usually also carriedout in the presence of an inhibitor which inhibits the polymerization ofthe carboxylic acid and/or of the ester. Particularly suitableinhibitors are hydroquinone, hydroquinone monomethyl ether,p-methoxyphenol, p-benzoquinone, phenothiazine,4-hydroxy-2,2,6,6-tetramethyl-1-oxylpiperidine and methylene blue, whichare used for stabilization in amounts of from about 200 to 2,000 ppm,based on the weight of the starting materials. The novel process ishowever not restricted to specific catalysts or inhibitors, and theacid:alcohol ratios, the pressure prevailing during the esterificationand the reaction time also play just as minor a role. A typical molaralcohol:acid ratio is from about 1:0.7 to 10 1:1.2, and typical reactiontimes are from about 1 to 10, preferably from about 1 to 6, hours. Theesterification can be carried out at atmospheric, superatmospheric orreduced pressure and both continuously and batchwise.

[0016] Suitable apparatuses for carrying out the esterification andisolating the desired ester from the reaction mixture are conventionalunits as described, for example, in Ullmann's Encyclopedia of IndustrialChemistry, 5th Ed., 1985, Volume A1, pages 168, 169, and Kirk-Othmer,“Encyclopedia of Chemical Technology”, Vol. 1, pages 341, 342.

[0017] Alcohol, acid, catalyst and, if desired, polymerization inhibitorare introduced continuously or batchwise into a reactor, for example asimple heatable kettle.

[0018] The olefin on which the alcohol used is based (or the parentolefins) is (are), as mentioned, formed as a byproduct in theesterification and the working up.

[0019] The amount of olefin formed is dependent on the reactionprocedure and the starting substances. In continuous operation, theamount of olefin is, if required, kept constant by removal.

[0020] During the formation of the olefins by elimination of water fromthe alkanol used for the esterification, by cleavage of the ester underthe esterification conditions or by cleavage of the oxyesters, in manycases it is not only an olefin which is formed but, as a result ofisomerization, a mixture of two or more olefins, depending on thealcohol used. For example, when the alcohol used is 2-ethylhexan-1-ol,it is not only 2-ethylhex-1-ene which is formed but a mixture ofisomeric octenes which contains 2-ethylhex-2-ene and 3-methylhept-2-eneas main components. The proportion of the components depends on thereaction conditions.

[0021] Accordingly, according to the invention the term “the olefin onwhich the alcohol is based” is to be understood as meaning not only a(single) olefin but also a mixture of two or more isomeric olefins.

[0022] The azeotropic mixture distilled off from the reactor andcontaining the olefin as entraining agent may also contain certainamounts of desired ester and/or starting alcohol and/or starting acid.The amounts are dependent on the reaction procedure and on the type ofacid used and on the alcohol used. The water separates out from theazeotropic mixture distilled off and is removed. The organic phase whichcontains the olefin can be recycled together with the other componentsof the mixture, if present, directly to the esterification reaction.This feed can be effected continuously or batchwise. The water which isseparated out can be subjected to further working up in order to obtain,for example, desired ester contained therein and/or to recover startingcompounds, which can be recycled to the esterification reaction.Moreover, the aqueous phase can be used as wash liquid in one of thewash processes which may be provided for working up the esterificationmixture. If desired, the olefin phase too can be treated in order toremove ester or other products contained therein before the olefin isrecycled to the esterification reactor. However, the simplest and mosteconomical and therefore preferred procedure comprises directlyrecycling the entraining agent together with impurities containedtherein, such as desired ester and/or starting alcohol and/or startingacid, possibly also with other byproducts, to the esterificationreactor.

[0023] The esterification reaction mixture in the novel process foresterifying an α,β-unsaturated carboxylic acid is worked up in aconventional manner, i.e. unconverted starting compounds and the desiredester are separated from the reaction mixture by distillation, the acidcatalyst used for the esterification being separated off, if necessary,beforehand by extraction by means of water and/or aqueous alkali (cf.for example Ullmann's Encyclopedia of Industrial Chemistry, Vol. A1, 5thEd., VCH, page 167 et seq.). The working up can be carried out after theend of the esterification reaction, but it is also possible to removereaction mixture continuously and to work it up. The bottom productremaining during the working up of the esterification reaction mixtureby distillation contains the oxyesters. The bottom product can besubjected to a treatment for recovering starting substances and/or forobtaining desired ester, the process described in DE-A-195 36 191preferably being used. The olefin obtained can be used as an entrainingagent. The cleavage products are preferably recycled to theesterification reactor.

[0024] In the process according to DE 195 36 191, the bottom product tobe cleaved is removed continuously or batchwise from the working up ofthe esterification mixture by distillation and is fed with the cleavagecatalyst to the cleavage reactor. Also possible is a semicontinuousreaction procedure in which the product to be cleaved is fedcontinuously to the cleavage reactor which contains the cleavagecatalyst, and the bottom product is removed batchwise from the cleavagereactor only after the end of the cleavage. The cleavage products areseparated off continuously by distillation. The cleavage is preferablycarried out in the presence of acid at from about 150 to 250° C. and ata pressure such that the cleavage products formed from the oxyestersvaporize. Preferably, the process is carried out in the presence ofmolecular oxygen. Regarding the exact manner for carrying out theprocess, reference may be made to DE-A-19 536 191.

[0025] The cleavage products contain, in addition to the desired ester,starting carboxylic acid and starting alcohol, a considerable amount ofolefins, for example up to about 20% by weight of octenes in the case ofthe cleavage of the residue from the preparation of 2-ethylhexylacrylate. These octenes are essentially a mixture of isomeric octenescontaining 2-ethylhex-2-ene and 3-methylhept-2-ene as main components.

[0026] The olefin formed in the oxyester cleavage can be fedcontinuously or batchwise to the esterification process. Prior removalof the further oxyester cleavage products, i.e. desired ester, startingalcohol and starting acid, is not required but can be carried out. Onepossibility is to separate the monomeric ester from the mixturevaporizing in the oxyester cleavage and containing the cleavage productsand to recycle the remaining cleavage products, i.e. olefin, startingacid and starting alcohol and any further cleavage products to theesterification reactor. Consequently, the monomeric ester is preventedfrom undergoing a possible further reaction, the recovered startingmaterials are recycled to the reaction to increase the yield, and theolefin is used as an entraining agent for water of reaction.

[0027] The novel process is not restricted to specific α,β-unsaturatedcarboxylic acids, but monounsaturated or polyunsaturated carboxylicacids and mono- or dicarboxylic acids are suitable. However, the novelprocess is preferably used in the case of monounsaturated monocarboxylicacids of 3 to 6 carbon atoms and in the case of monounsaturateddicarboxylic acids of 4 to 8 carbon atoms, e.g. acrylic acid, crotonicacid, maleic acid, fumaric acid, itaconic acid, etc. The novel processis particularly advantageously used in the case of acrylic acid andmethacrylic acid and in particular for the preparation of 2-ethylhexylacrylate.

[0028] Preferred alkanols are those of 6 to 10 carbon atoms and inparticular 2-ethylhexanol (2-ethylhexan-1-ol) or 2-propylheptanol.

[0029] It has surprisingly been found that, in spite of the fact thatthe formation of esters is actually to be expected, olefin isomermixtures can be used as entraining agents without the esterification andthe quality of the desired ester being influenced. A typical example ofsuch an isomer mixture is the stated mixture of isomeric octenescontaining 2-ethylhex-2-ene and 3-methylhept-2-ene as main components.

[0030] The novel process has the advantage that it is simpler to carryout because no “foreign” entraining agent is used, which introduces anadditional component into the system. Rather, an olefin which is in anycase formed as a byproduct in the esterification is used as theentraining agent. There is therefore no longer any need specifically toseparate off this byproduct, which has a lower boiling point than thedesired ester.

[0031] The invention is illustrated below with reference to examples,which however serve merely for illustration and in no way are to beunderstood as imposing any restriction:

[0032] The acrylic acid used in the examples below contained essentiallythe following components: Acrylic acid 99.7% Acetic acid 0.10% Propionicacid 0.04% Diacrylic acid 0.08% Water 0.05% MEHQ (hydroquinonemonomethyl ether) 0.02%

[0033] The entraining agent used was an octene mixture which originatedfrom the cleavage of the oxyesters obtained in the preparation of2-ethylhexyl acrylate. According to gas chromatographic analysis, itcontained the following main components: 2-Ethylhex-2-ene   32%3-Methylhept-2-ene 46.7% 5-Methylhept-2-ene 14.3%

EXAMPLE 1

[0034] 336 g of an octene mixture, 238 g of acrylic acid, 1.2 g ofphenothiazine and 5.74 g of sulfuric acid (98% strength) were heated ina stirred reactor while stirring and were kept at 114-117° C. for 5hours. After neutralization of the reaction mixture, no octyl acrylateswere detectable by means of gas chromatography.

EXAMPLE 2

[0035] A mixture of 238 g of acrylic acid, 390 g of 2-ethylhexanol, 190g of octene mixture, 0.5 g of phenothiazine and 6.28 g of sulfuric acid(98% strength) was heated to the boil with stirring and underatmospheric pressure in a stirred reactor with attached distillationcolumn (30 cm, 0.5 cm Raschig rings), condensater and water separatorand was refluxed for 1.5 hours. The temperature in the reactor increasedfrom 116° C. to 159° C. 50 g of water was separated off via the waterseparator. The residual acrylic acid content in the reaction productmixture was 3.1%. The acrylic acid conversion was 99% of theory. Gaschromatographic analysis showed that no isomeric octyl esters wereformed.

EXAMPLE 3

[0036] The procedure was as in Example 2, except that cyclohexane wasused instead of the octene mixture. The required reaction time was 2.5hours. The reaction temperature increased from 98° C. to 114° C. Theacrylic acid content of the final mixture was 3.2% and the acrylic acidconversion was 99% of theory.

We claim:
 1. A process for the preparation of an ester of an α,β-unsaturated carboxylic acid by reacting the carboxylic acid with an alcohol which is selected from C₆-C₁₂-alkanols, cyclopentanol and cyclohexanol, in the presence of an acidic esterification catalyst and of an entraining agent for removing the water of reaction formed in the esterification, wherein the entraining agent used is the olefin on which the alcohol is based.
 2. A process as claimed in claim 1, wherein olefin and water are removed from the reaction mixture as a mixture by distillation, water is separated off from the mixture and the olefin is recycled to the esterification process.
 3. A process as claimed in claim 1 or 2, wherein, after the end of the reaction, the volatile components are separated off from the reaction mixture and the remaining, oxyester-containing product is worked up, the oxyesters being cleaved and the cleavage product, if necessary after removal of the ester contained in the cleavage product, being recycled as entraining agent to the esterification process.
 4. A process as claimed in any of claims 1 to 3, wherein the alcohol used is a linear or branched C₆-C₁₀-alkanol.
 5. A process as claimed in claim 4, wherein the alcohol used is 2-ethylhexan-1-ol or 2-propylheptanol.
 6. A process as claimed in claim 5, wherein the entraining agent used is a mixture of isomeric octenes containing 2-ethylhex-2-ene and 3-methylhept-2-ene as main components.
 7. A process as claimed in any of claims 1 to 6, wherein the α,β-unsaturated carboxylic acid used is a monounsaturated monocarboxylic acid of 3 to 6 carbon atoms.
 8. A process as claimed in any of claims 1 to 6, wherein the α,β-unsaturated carboxylic acid used is a monounsaturated dicarboxylic acid of 4 to 8 carbon atoms.
 9. A process as claimed in claim 7, wherein the α,β-unsaturated carboxylic acid used is acrylic acid or methacrylic acid. 